Effect of Methyl Parathion on the Growth and Reproduction of Eisenia fetida in Natural Soil

Abstract

A laboratory study was carried out to assess the toxic effects of organophosphate insecticide methyl parathion (MP) 2% dust (active ingredient, a.i.) on epigeic earthworm Eisenia fetida at different lethal and sub-lethal concentrations. A series of toxicity tests starting from range finding acute to chronic were conducted in natural soil to determine the sensitive endpoints like survival rate, growth rate and reproduction rate of E. fetida. Results from the study revealed a reduction in the earthworm biomass and reproduction potential. The median lethal concentration (LC50) determined from 14-day acute toxicity test was 36.63 mg a.i./kg, which confirmed the extremely toxic nature of MP 2% dust. The addition of septic tank sludge as a feeding material in the chronic toxicity test helped the earthworms in gaining biomass at lower doses of 2.5 and 4.0 mg a.i./kg of MP 2% dust. At a lower concentration in the range of (6.4–26.2 mg a.i./kg), MP 2% dust showed sub-lethal toxicity to E. fetida. Change in the earthworm biomass and reproduction rate during the toxicity test appeared to be the more sensitive endpoints compared to the LC50 value used for assessing the earthworm mortality from pesticide. This study provides an insight into the ecological risks associated with pesticides and helps to provide an understanding of the sustainable use of pesticides for ecological protection.